Territorial describes the strong instinct to claim, protect, and defend a specific area as your own. Animals are often territorial: a dog might bark at strangers who approach its yard, or a bird might chase other birds away from the tree where it built its nest. The animal feels that this space belongs to me, and acts to keep others out.

People can be territorial too. If you have a younger sibling who keeps wandering into your room and messing with your things, you might feel territorial about your space and want to establish boundaries. Some students get territorial about their usual seat in class, feeling annoyed when someone else sits there. Countries are territorial about their borders, which is why maps show where one nation ends and another begins.

When you're being territorial, you're treating something like it's your territory, whether it's actual space, a project you're leading, or even your place in line. Being territorial isn't always bad (you have a right to privacy in your own room), but taken too far, it can make you seem unwilling to share or work with others.